Physical interface modules, such as the ‘enhanced Small Form-factor Pluggable’ module (SFP+), the ‘Quad Small Form-factor Pluggable’ module (QSFP) and the ‘120 Gb/s 12× Small Form-factor Pluggable’ module (CXP), are compact, hot-pluggable transceivers used for both telecommunication and data communications applications. Physical interface modules are typically used to interface a network device to a fiber optic or copper networking cable. For example, an SFP+ module is typically inserted into an SFP+ cage on an electronic device, such as a server or a switch.
Physical interface modules are hot-swappable electronic components that have an electrical interface toward the electronic device being interfaced with the network, and a specific copper or optical interface toward the network. Physical interface modules are widely used because of their hot-swappable characteristic, which also means that they are replaceable at run-time. While preserving the same electrical interface with the electronic devices, several different copper or optical interfaces can be used toward the network. Such flexibility has resulted in wide adoption of physical interface modules.
There are different variants of physical interface modules, mainly depending on the required bandwidth, speed, power and distance of the interconnection.
While there are several variants of physical interface modules, they typically all have the characteristic of being pluggable at the edge of electronic cards, boards or boxes. Once the physical interface module is inserted on the edge of the device, a networking cable can be connected to the physical interface module. For example, a physical interface module can be inserted in an MSA (multi-source agreement) compliant module cage located on a PCB (printed circuit board). Each PCB may have an MSA-compliant module cage holding a physical interface module.
Similar to the concept of physical interface modules, an active optical cable (AOC) is located on the edge of an electronic card, board or box. An AOC typically reuses the same electrical interface as a physical interface module, such as a QSFP module, as well as the same module cage typically located on the electronic equipment. One of the main differences between a physical interface module, such as a QSFP module, and an AOC, such as an AOC-based QSFP, is that the AOC does not provide a standardized networking interface, only a standardized electrical interface toward the electronic device. For example, in the case where an AOC vendor provides an optical signal at a specific non-standardized wavelength, then only that vendor can interconnect with its own equipment. Since an AOC vendor needs to provide the cable and both terminations of the cable, the technologies which can be used between both ends of the cable can be vendor specific.
Both ends of an AOC are terminated with a physical interface module. The modules themselves cannot be separated from the cable. Two PCBs can be connected using an active optical cable. A standard compliant MSA cage is installed on each PCB. The AOC has connectors at both ends, which includes an optical engine in each connector. The optical engines are responsible for converting signals between the electrical and optical domains. In order to interconnect the two PCBs, an active optical cable is used, where each end of the cable is inserted in each module cage of the PCBs.
The module cage is a cage where a compliant physical interface module or AOC can be inserted. The cage can be used as a guide toward the backend connector of a physical interface module or AOC, in order to interface the PCB upon insertion. The connector typically uses electrical traces, where data, control and power are communicated.
By using standardized physical interface modules and AOCs, electronic equipment such as servers, switches and routers can be developed with the option of leaving to the network operators the task of carefully selecting the required network interface at deployment time. While such an approach has some advantages, there are a few optimizations that could be considered in order to better address the increasingly important challenges concerning footprint and energy consumption.
For example, a large part of the energy consumed by a physical interface module located on the edge of a board, such as a CXP module or an AOC, is used to interface the electronic components directly located on the same board. This means that modules located on the edge of cards or boards could greatly reduce their power consumption if the length of electrical traces between electronic components on PCBs, such as ASICs (application-specific integrated circuits), and a physical interface module located on the edge of the PCBs were significantly shortened.
Also, physical interface modules are typically designed for flexibility and interoperability, resulting in their form-factor not necessarily being optimized in terms of size. For example, the same QSFP physical interface module specification can be used for short-reach and for long-reach applications. While the long-reach variant requires significantly more power than the short-reach variant, the specification of the form-factor for the QSFP module is typically based on the required power consumption of the long-reach variant. As such the size of a physical interface module is typically much larger than what could be minimally required.
Recently a new type of electronic component has received more widespread use for interfacing electronic equipment: optical engines. Optical engines are components used to convert electrical signals into optical signals, and vice-versa. The way in which optical engines are typically built requires them to be placed in very close proximity to the source of the electrical signals being converted into optical signals. While the close proximity minimizes the length of electrical traces, the complexity of electronic components typically required in CXP modules and AOCs can be greatly reduced, such as eliminating the need for Clock Data Recovery (CDR) functions. That reduced complexity brings smaller footprint and power savings.
Optical engines can be extremely small, and in comparison with an equivalent standardized physical interface module, optical engines can be smaller by an order of magnitude. While there are several different vendor-specific variants of optical engines and no standardized form-factor and vendor agreement for building optical engines, optical engines are aimed at providing extremely efficient and optimized solutions in terms of energy consumption and footprint. The lack of standardization for optical engines can result in functional incompatibility and incompatible optical engine form-factors.
Optical fiber connectors are also typically used at the edge of a network. An optical fiber connector terminates the end of an optical fiber. A connector mechanically couples and aligns the cores of fibers so that light can pass. Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. In telecommunication and data communications applications, small connectors, e.g., so-called LC, and multi-fiber connectors, e.g., so-called MTP, are replacing more traditional connectors (e.g., so-called SC), mainly to provide a higher number of fibers per unit of rack space.
A type of MT connector, e.g. an MTP or MPO connector, can be used to interconnect up to 72 optical waveguides. The alignment of the optical fibers is possible because of the alignment pins available on the MT connectors. In the case where multiple MT connectors are required, an array connector can be used to hold several MT connectors. For example, each MT connector allows up to 72 optical channels. While each MT connector has alignment pins, additional alignment pins for the array connector itself are also provided. As such, an array connector can be very large.
Each of the network edge components described above have disadvantages. For example, currently network functions are only provided directly on PCBs or on modules located on PCBs and thus additional space is required on a PCB layout for supporting networking functions. Currently, it is not possible to cascade pluggable interface modules. Also pluggable modular network functions, such as optical switching and wavelength conversion, do not exist presently. When needed, interface functions are included in more complex interface modules to provide tailored solutions, which typically lack flexibility and offer limited capabilities. Modules that include more network functions than minimally required typically result in significantly larger modules which are more expensive and less energy efficient. Such modules also do not interface with other similar modules. Legacy MSA cages represent a waste of space when the latest technologies provide an equivalent functionality in much less space. While optical engines are gaining popularity for network interconnections, the lack of standardization for optical engines can result in functional incompatibility and incompatible optical engine form-factors. Moving forward, optical backplanes or interconnects will become more important and require network functions inside the backplanes themselves, or interconnects. It is not possible today to provide network functions on backplanes with a pluggable network interface module concept.